The present invention relates to a pattern developing process and an apparatus therefor, which can detect completion of resist development.
In recent years, many highly integrated, high-speed semiconductor devices have been developed. This has required high-precision micropatterning of various circuits formed on semiconductor wafers.
In order to satisfy such a requirement, various improvements have been made. For example, as regards the lithography technique, extensive studies have been made about use of short wavelength X-rays instead of conventional ultraviolte ray as a light source when a resist is exposed through a mask. In a method of manufacturing a mask, the conventional method of exposing a pattern with ultraviolet ray is being replaced with a method of exposing a pattern with electron beams which allows micropatterning.
A method of manufacturing a photomask or an X-ray mask by electron-beam lithography will be described. A metal film is formed by deposition or sputtering on a substrate which is transparent in a wavelength range of an exposure light source. Next, an electron-beam resist is coated on the metal film, and a desired pattern is exposed with electron beam. Thereafter, the drawn pattern is developed to selectively remove the electron-beam resist, thus forming a resist pattern. The metal film is etched using the remaining resist pattern as a mask to form a desired mask pattern. Thereafter, the resist pattern is removed to form a mask.
An electron-beam resist coated on a semiconductor wafer can be directly exposed with electron beam without using a lithography technique, thus obtaining a still finer pattern. In this electron-beam exposure technique, the electron-beam resist is coated on the semiconductor wafer and a desired pattern is exposed thereon by use of electron beam. Next, the exposed pattern is developed to selectively remove the electron-beam resist, thus forming a resist pattern. A semiconductor layer or metal film on a semiconductor substrate is etched or doped using the remaining resist pattern as a mask to form a desired circuit pattern and element. Thereafter, the resist pattern is removed.
However, when a micro mask pattern is formed on the electron-beam resist as described above, the developing temperature greatly influences the developing speed, and therefore, the period of the developing step. The developing period is set based on experience, but the period thus set is not always the optimum. This is because the developer contains an inorganic solvent such as ketone or alcohol, which evaporates from the surface of the developer in a developing tank and deprives heat, thus making the temperature distribution of the developer nonuniform. Hence, the developing step must be repeated to accurately determine the actual developing time. Even though a proper time can be determined through repeated tests, if the developer temperature varies widely during such tests, this renders the test results inaccurate. The developing rate is also influenced by the dose of electron beam, variations among manufacturing lots of resist, degradation in developer, baking conditions after coating the resist, and the like. Therefore, although the developing step is performed for a predetermined period of time, a wafer or a mask is often underdeveloped or overdeveloped, and the size of the formed pattern may be greatly differnt from design values and cannot therefore meet the required level of precision.
In order to solve this problem, the developing time is gradually changed by an operator while repeating the developing step. However, since the variation factors (e.g., the temperature of the developer) are not always constant, this method has low reliability and poor reproducibility, and does not allow formation of patterns with high precision.
In addition, a technique for determining completion of development, which utilizes a change in reflectance of laser beams radiated on a resist pattern during development, has been studied. However, such an optical method is influenced by the refractive index of the developer and scattering of the laser beam in the developer, thus resulting in poor reliability.
Another method for determining the developing end point has been described in PCT application No. WO81/00646. In this technique, a wafer and an electrode which is kept at a constant potential by a bias means and is connected to the wafer, are dipped in a conductive developer, and an insulating resist is removed by the developer. Then, the completion of pattern development is determined from a change in voltage when the electrode and the wafer are electrically connected and a circuit is formed. More specifically, electrical connection is detected when the voltage exceeds a predetermined threshold value. However, this technique simply utilizes the Ohm's law where, when electrical resistance of the resist is decreased and the circuit is formed, a current flows, and which is converted into voltage. The voltage rises abruptly when the electrode and the wafer are electrically connected. However, this change in voltage depends on types of wafer, and it is difficult to determine the completion of development, regardless of the type of the wafer. In this technique, when the voltage exceeds a predetermined threshold value, the completion of development is detected. However, such a method can hardly set an optimal developing time for each wafer. For this reason, in this technique, the developing time for each different type of wafer to be mass-produced is corrected using test wafers. This results in a time-consuming process and it is difficult to obtain a desired patteren size with high precision.